An electrorheolgical fluid whose viscosity greatly, instantaneously, and reversibly changes on voltage application has been well known since the 1940's as a so-called Winslow fluid, comprising an insulating oil having dispersed therein water-containing fine particles, such as silica and starch (see U.S. Pat. No. 2,417,850).
Since then, many improvements on electrorheological fluids have hitherto been proposed. For example, it has been proposed to use, as water-containing fine particles, ion exchange resin particles (see JP-A-50-92278, the term "JP-A" as used herein means an "unexamined published Japanese patent application") or zeolite particles (see JP-A-2-3711), or to use non-water-containing particles, such as organic semi-conductor particles (see British Patent 2,170,510), conductor particles having the surfaces thereof rendered insulating (see JP-A-64-6093), and liquid crystal polymer particles (see Proceedings of the 2nd Int'l Conf. on ERF, p. 231 (1989)). These electrorheological liquids containing fine particles exhibit excellent performance properties for a short period of time but unavoidably undergo sedimentation and flocculation of dispersed particles and become muddy with the passage of time. They have thus not seen practical use.
On the other hand, homogeneous liquids containing no particles which are now under study include polar liquids, e.g., nitromethane and nitrobenzene (see Japan J. Appl. Phys., Vol. 16, p. 1775 (1977)), cholesteric liquid crystal mixtures (see Communication, p. 3865 (1965)), low-molecular liquid crystals, e.g., methoxybenzylidenebutylaniline (MBBA) (see Japan J. Appl. Phys., 171525 (1978) and British Patent 2208515A), and ferroelectric polymer solutions (see Dai 39-kai Kobunski Toronkar Yokosyu, 18U07 (1990)). However, these liquids reveal no substantial electrorheological effect.
Furthermore, for operation these conventional electrorheolgical fluids require the application of high voltages of 2 to 3 kv/mm. Such electrotheological fluids are limited to use at low temperatures since current value increases rapidly at high temperatures.
Liquid crystals undergo molecular orientation when a voltage is applied to them to cause anisotropy in various characteristics. For example, a change in elasticity has been reported, with respect to a lyotropic liquid crystal with a voltage applied thereto (see Dai 16-kai Ekisyo Toronkai Koen Yokosyu, 1K216 (1990)).
Liquid crystals have therefore been contemplated for use as electrorheological fluids, but little importance has attached thereto due to the smallness of viscosity increase observed.